Wind turbine generators are frequently equipped with gearboxes that increase shaft speed of the rotor blades to an output shaft speed to the generator by step up ratios as high as 100 to 1. For example, if the rotor blades are rotating at 20 RPM, the output shaft of the gearbox might be 2000 RPM. The speed and angular acceleration produced create a varying and difficult set of dynamic conditions for the output shaft. Output shafts (as well as the shafts they mesh with) are generally parallel gear shafts equipped with helical gears producing radial and axial loads that must be supported by the bearing system. The locating bearings that fix the axial location of output shafts have had several configurations in the past. The gearbox shown in FIG. 1 shows a combined NU style cylindrical roller bearing RB and 4-point contact ball bearing BB. A 2-row spherical roller bearing or a 2-row tapered roller bearing in a direct style mounting (commonly referred to as X configuration) has also been used. In particular, the combination bearing assembly of FIG. 1 and the spherical roller bearing styles mentioned above experience performance problems manifested with smearing of raceway surfaces, micropitting and retainer failure.
Previous attempts at solving the problems on these parallel shaft location positions include use of 2 single-row tapered roller bearings adjusted against each other in a “cross-locating” configuration. The objection to this solution is that close control of adjustment is critical and not easy to obtain reliably. Preload would be desirable but speeds and heat generation require the initial setting to have clearance. Another solution is to fix a 2-row spherical roller bearing that eliminates the need for on-sight adjustment of clearances, but this style bearing experiences damage to raceways and retainers because it is not a particularly good bearing style for combined radial and thrust loading when the proportion of thrust load is high and/or reversing. The dynamics of torque reversing from positive (during power generation) and negative (during motoring) produces excessive roller sliding/skidding and these forces strain the retainers. More recent solutions use 2 row spring loaded 2 row tapered roller bearing assemblies mounted in an X configuration at the locating position. This solution can work successfully but requires loose fitted outer races in the housing that should be keyed in place to prevent rotation under load. These configurations also require careful control at assembly to insure the springs have been deflected properly prior to operation.